1. Field of the Invention
The invention is related to integrated circuits and integrated circuit technology, and in particular, to integrated circuit low noise/analog power supplies.
2. Background Information
Fluctuations (or interference) in the core voltage supply of conventional microprocessors are filtered out using an inductor-capacitor circuit. In general, the inductor-capacitor circuit filters the core voltage supply to generate an analog voltage supply.
FIG. 1 shows a block diagram of a typical circuit 100 used to filter a microprocessor core voltage supply 102 and generate an analog voltage supply 104. The circuit 100 includes an inductor-capacitor (LC) filter 106, which is a low pass filter. This means that the LC filter 106 when operating as desired, allows low frequencies to pass through it and attenuates high frequencies. The LC filter 106 includes an inductor 108 and a capacitor 110. The analog voltage supply 104 is coupled to a phase locked loop (PLL) circuit 112. The PLL circuit 112 is located on a microprocessor 120. The return path for the analog voltage supply 104 is a return path 114.
One limitation of the circuit 100 is poor frequency performance by the LC filter 106, and as a result, for the circuit 100 overall. This is because the inductor 108 has parasitic capacitance and the capacitor 110 has parasitic inductance. FIG. 2 is a graphical representation a response curve 200 for the LC filter 106, which shows a pole 202 at fifteen kilohertz (kHz), where the gain of the circuit 100 is reduced by approximately 3 dB. This means that at fifteen kHz the gain of the circuit 100 is half of what the gain is at zero hertz.
Also shown in FIG. 2 is a notch 202 at one megahertz (MHz). At frequencies higher than or equal to one MHz, the gain of the circuit 100 increases significantly, which is the opposite of the desired frequency performance.
The response curve 200 also shows a reflection portion 204. The reflection portion 204 indicates that at frequencies higher than or equal to one Mffz the LC filter 106 begins to pass high frequencies, which is undesirable.
FIG. 2 also shows another limitation of the circuit 100, which is the noise amplification at Fpeak 206 (or peak frequency) due to the second order nature of the LC filter 106. The noise amplification may degrade the phase noise performance of the PLL circuit 112.
A further limitation of the circuit 100 is that when the core voltage supply 102 changes the analog voltage supply 104 to the PLL circuit 112 changes accordingly. For example, operation in wide ranges of variations in the analog voltage supply 104 may degrade the PLL circuit 112's performance. Wide ranges in the analog voltage supply 104 also may cause the PLL circuit 112 to cease operating.
Another limitation is that each phase locked loop circuit have its own inductor-capacitor filter. This means that as the number of phase locked loop circuits increases the number of LC filters, and individual capacitors and inductors, increases.
Moreover, the prior art LC filter is typically located on a computer's motherboard. This can mean a large number of components on each motherboard, depending on the number of PLL circuits 112 in a particular processor.